ε-Fe2O3 is an extremely rare phase in iron oxides, and since particles having a size of the order of nanometers show a huge coercive force (Hc) of approximately 20 kOe (1.59×106 A/m) in room temperature, a production method in which ε-Fe2O3 is synthesized in a single phase has been conventionally studied (PTL 1). When ε-Fe2O3 is used in a magnetic recording medium, there is no magnetic head material having a saturation magnetic flux density of a high level responding thereto at this time. Accordingly, control of the coercive force is being conducted by substituting a part of Fe sites of ε-Fe2O3 with a trivalent metal such as Al, Ga, and In, and the like, and the relation between the coercive force and the radio wave absorption characteristic is being analyzed (PTL 2).
On the other hand, in the field of magnetic recording, a magnetic recording medium having a high ratio of reproduced signal level and particulate noise (C/N ratio: Carrier to Noise Ratio) is being developed, and for increasing recording density, reduction of the size of the magnetic particles forming a magnetic recording layer is demanded. In general, however, the reduction of the size of the magnetic particles is likely to cause deterioration in environmental stability and thermal stability, and there is a concern of deterioration in magnetic characteristics of the magnetic particles in the use and storage environments. Therefore, by substituting a part of Fe sites of ε-Fe2O3 with another metal superior in heat resistance, there are developed various kinds of partially substituted product of ε-Fe2O3 which are represented by the general formula ε-AxByFe2-x-yO3 or ε-AxByCzFe2-x-y-zO3 (wherein, A represents a divalent metal element such as Co, Ni, Mn, and Zn, B represents a tetravalent metal element such as Ti, and C represents a trivalent metal element such as In, Ga, and Al), which make it possible to reduce the particle size and allow the coercive force variable, and which is also superior in environmental stability and thermal stability (PTL 3).
Since ε-Fe2O3 is not a thermodynamically stable phase, a specific method is required for producing the same. PTLs 1 to 3 mentioned above disclose a production method of ε-Fe2O3 in which fine crystals of iron oxyhydroxide produced by a liquid phase method are used as a precursor, and after coating the precursor with a silica by a sol-gel method, the precursor is subjected to a heat treatment, and as the liquid phase method, a reverse micelle method in which an organic solvent is used as a reaction medium, and a method in which only an aqueous solution is used as a reaction medium are disclosed.